The present invention relates to a discharge lamp, more specifically to a highly stable, long lifetime short-arc discharge lamp to be utilized for light exposure apparatus for fabrication of semiconductor integrated circuits (ICs)
IC fabrication processes require various technologies including a light exposure technology (or photolithography). A special resin to be hardened by exposure of ultra violet ray is used to form a desired pattern on a semiconductor substrate. The exposure apparatus to be used for fabrication of such electronic devices is generally short-arc discharge lamps. In particular, as the recent ICs are highly integrated, discharge lamps for efficiently irradiating approximately 365 nm short wavelength i-ray are mostly used for such applications.
Since IC technologies advance very rapidly, IC fabrication requires a large capital investment and encounters severe cost competition. It is therefore essential for the success of IC manufacturers to reduce production cost. Also, there are strong needs for reducing the running cost by extending the lifetime of a short-arc discharge lamp that is one of key consumption equipment. Conventional ways of extending lifetime of a short-arc discharge lamp include suppressing the reduction in intensity by improving shapes and treatments of various constituent elements or materials of the electrodes, or improving i-ray irradiation efficiency to increase the intensity on the exposure surface.
A short-arc discharge lamp utilizing i-ray for an IC exposure apparatus typically uses approximately xc2xdxcx9c{fraction (1/10)} mercury content to be sealed in the lamp as compared with a short-arc discharge lamp to be normally used in an exposure apparatus for liquid crystal display panels (LCDs) and printed circuit boards (PCBs) in order to enhance the i-ray irradiation efficiency. This tends to decrease the internal pressure in the discharge tube. A common approach to overcome the decreased irradiation intensity due to decreased internal pressure is to increase rare gas pressure sealed in the discharge lamp as a buffer. This is effective to suppress wear of the electrode and also decrease the intensity maintenance rate.
In the exposure apparatus, the radiated i-ray is not only used for the exposure purpose but also for focus control in exposure, thereby requiring highly stable intensity. In more recent years, shorter wavelength light sources such as excimer lasers are used. This makes it effective to expose light while scanning on a substrate, thereby increasing the needs for high intensity stability.
It was impossible to sufficiently meet the needs for extend lifetime of the light source for exposure apparatus. It is therefore an object of the present invention to provide a short-arc discharge lamp having extended lifetime, high intensity efficiency and stabilized intensity.
In order to overcome the above problem and to achieve the above object, the short-arc discharge lamp according to the present invention comprises a discharge tube having a cathode and an anode disposed within the tube in an opposed relationship and mercury and rare gasses sealed inside the tube. It features in the sealed rare gases comprising a mixture of a high molecular weight rare gas such as xenon (Xe), krypton (Kr) or argon (Ar) and 5%xcx9c40% by volume of a low molecular weight rare gas such as helium (He) or neon (Ne). And the pressure of the mixed rare gases is set to 2 atmospheric pressure or higher at normal temperature.
The anode comprises a substantially columnar body portion, a tapered portion and a flat end portion. The distance L (mm) of the point having the current density of 0.3A per square mm from the flat portion is set to 0.051 less than L less than 0.081, where I (A) is the discharge current. The diameter of the flat portion at the front end of the anode is set to 2 times or less the distance between the cathode and the anode. And the current density of the anode is set to 3.5A or less per square mm.